In order to protect public health, the United States Environment Protection Agency (EPA) has developed air sampling protocols, including a standardized air sampling system. Under such EPA protocols, in order to collect a series of daily samples a site operator needs to be at an air monitoring site that is equipped with a single event air sampler every day so as to retrieve the sample-loaded filter and install a new filter.
To minimize inconvenience, many monitoring sites have accommodated sequential air samplers. Currently most sequential samplers use a linear transfer movement mechanism to move successive filter cassettes from a clean filter cassette magazine station to a sampling station and finally to a storage magazine station. For this sequential movement, a filter cassette carrier moves forward and backward by precisely controlled actuators.
Some systems use several electric linear actuators, stepper motors with belt, or pneumatic actuators to achieve this linear movement. Therefore, these systems often require an accurate control system to correctly position the carrier.
One broadly available EPA designated sequential air sampler uses a pneumatic actuator and solenoid actuators to move filter cassette from position to position. To activate the pneumatic actuators, lots of complex pressurized air tubes are linked with valves and other control systems. Therefore, the biggest drawback of this system is maintaining leak-free complex pneumatic system. If a malfunction occurs, it would require a lot of time and effort to find the source(s) of the malfunction.
U.S. Pat. No. 5,898,114 to Basch et al. uses a multi-filter cassette carrier for a sequential air sampler. In U.S. Pat. No. 6,138,521 to Basch et al., the cassette magazines are installed beneath the transfer mechanism. Therefore, it requires a filter cassette lifting mechanism at the supply station and also requires a drop preventing aperture at the storage magazine station. This system uses a pneumatic actuator and couple of solenoid actuators to move filter cassette from position to position, and therefore require precise pneumatic and sequential control and frequent maintenance.
The system in U.S. Pat. No. 8,192,516 to Yoon et al. uses two step motors, one for moving filter cassettes from station to station and another for moving the filter cassette vertical position. This system also requires precise position control to locate the filter cassette at right positions.
U.S. Pat. No. 6,167,767 to Mengel et al. uses a Geneva drive for indexing sampler position.